Resist material for micro-fabrication

ABSTRACT

A resist material for micro-fabrication comprising a polymer in which the polymer backbone has thereon a moiety of the formula (I) ##STR1## wherein R represents a hydrogen atom, an alkyl group, an alkenyl group or an aryl group or an aralkyl group, the resist material being curable by electromagnetic radiation such as electron beams, X-rays or deep ultraviolet light with a wave length of less than about 3000A and being particularly suitable as a micro-fabrication resist material.

This is a division of application Ser. No. 926,067, filed July 19, 1978,now U.S. Pat. No. 4,273,858.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a micro-fabrication resist material which canreadily be polymerized and insolubilized and to methods of using thisresist material.

2. Description of the Prior Art

Micron order fabrication represented by IC's and LSI's is ordinarilyconducted by photo-lithography using a photo resist. However, ultra-finefabrication on a sub-micron order recently has been needed, making itimpossible to use photo-lithography employing photo-resist. Therefore,studies were begun on the resist material and on lithography utilizingelectromagnetic radiation with short wavelengths such as electron beamsand X-rays, replacing the use of near ultraviolet and ultraviolet light.Use of this resist material is similar to use of a photo-resist and theprocedure is described as follows. The substrate surface is coated withthe resist material and a thin layer of a polymer is formed on thesubstrate surface by evaporating the solvent. The coated substrate isthen exposed to electromagnetic radiation such as electron beams, X-raysor deep ultraviolet light and the polymer is cross linked and as aresult insolubilized in the exposed portion. When in the next step thesubstrate is developed with an appropriate solvent, the exposed portionof the polymer layer remains and the unexposed portion of the polymer iswashed away. The above-described resist material is usually categorizedas a negative-type resist material. The resist material of thisinvention is a negative type resist material. The substrate havingthereon the insoluble resist is subjected to doping, metallizing, andetching processing, etc. The fabrication process is completed byremoving the resist with a solvent or plasma ashing.

As is evident from the fabrication method described above, the resistmaterial must be have sufficiently high photoproperties such assensitivity, contrast, resolution, edge sharpness, must be adhesive tothe substrate and resistant to the ethchant and must have sufficientshelf life. Furthermore, the resist material should preferably not be sophoto-sensitive that it must be processed in dark room. The resistmaterial of this invention shows a high sensitivity to electromagneticradiation of a wavelength less than about 3000 A, such as electronbeams, X-rays, deep ultraviolet light (wavelength of approximately 2700A to 1500 A). It is of value to mention here that the hitherto knownnegative type resist materials are decomposed by deep ultraviolet lightirradiation resulting in a poor positive image. On the other hand, theresist material of this invention provides a very clear negative imageas shown in the examples given hereinafter.

Sensitivity of the resist material is indicated by the dose ofelectromagnetic radiation required to insolubilize the resist materialin the case of a nega-type resist material. In case of electron-beamsthat are most often utilized, contrast (γ) is attained at (γ=1 or above)supposing D^(i) as the minimum dose for starting insolubilization andD^(o) as a sufficient dose for 100% insolubilization. In order to permita resist material to be used practically, D^(o) ≦10⁻⁶ coulomb/cm² andcontrast≧1 are ideal. A study of lithography using electromagneticradiation (mainly electron beams) began in the last half of the 1960's.The resist material utilized initially was a common photo-resist. Aphoto-resist was an appropriate material for evaluation because itsbasic characteristics such as electro-sensitivity, adhesiveness to asubstrate such as a silicon wafer, coatability, and resistance toetchants were sufficiently high. However, the properties of such aphoto-resist were insufficient for electron beams, X-rays or deepultraviolet light lithography to be put into practice. For instance, thesensitivity of a negative type photo-resist to electron beams was low,i.e., D^(o) >10⁻⁵ and this low sensitivity was a decisive disadvantagefor spot scanning of electron beams which was used for mask productionwith a CAD. Around 1970, electron beam resist materials were announced.Among the many electron beam resist materials, a highly sensitive resistmaterial with a D^(o) approaching on the order of 10⁻⁸ coulomb/cm² wasdeveloped. Such, however, had inferior contrast and was totallyunsuitable for practical use. A photo-resist of another type, despiteits good sensitivity and contrast, did not become practical due todifficulty in synthesis. A photo-resist material for submicronfabrication which is suitable for practical use has yet to be developed.

SUMMARY OF THE INVENTION

In view of the current situation, materials suitable for lithographyusing electromagnetic radiation such as electron beams, X-rays or deepultraviolet light with a wavelength of 3000 A or less were investigatedand ultimately a resist material suitable for practical application wasdeveloped. Thus, the present invention provides a resist material withhigh sensitivity and contrast which can be readily polymerized andinsolubilized using electromagnetic radiation such as electron beams,X-rays or deep ultraviolet light. The resist material of the presentinvention suitable for micro-fabrication is not practicallyphoto-sensitive, and can be handled in the light, can be manufacturedwithout dificulty, and greatly contributes to development ofmicro-fabrication techniques due to its wide applications.

The present invention provides a resist material for micro-fabricationcomprising a backbone polymer having thereon a moiety of the formula (I)##STR2## wherein R is a hydrogen atom, an alkyl group, an alkenyl group,an aryl group or an aralkyl group, with the resist material having goodcoatability and being a solvent-soluble polymer.

DETAILED DESCRIPTION OF THE INVENTION

The moiety of the formula (I) above, hereinafter "1,2-ethylenicdicarboxylic moiety", can be advantageously introduced into a backbonepolymer using, a monoester of maleic acid, a monoester of fumaric acid,or maleic anhydride. The backbone polymer to which the 1,2-ethylenedicarboxylic acid derivative is bonded comprises repeating units of amonomer or monomers such as vinyl acetate, vinyl propionate, vinylchloride, methyl vinyl ketone, vinyl isocyanate vinyl pyrrolidoneacrylonitrile, ethyleneoxide, an alkyl acrylate, an alkyl methacrylate,butadiene, isoprene, acrylamide, styrene, itaconic acid, maleicanhydride and the like.

About 1% to 100% of the monomer units of the backbone polymer havethereon functional groups such as amino groups, hydroxyl groups andepoxy groups or ethylenically unsaturated bonds, etc. capable ofchemically bonding with the 1,2-ethylenic dicarboxylic moiety.

The description on the backbone polymer set forth above is given for thepurposes of illustration is not to be considered to limit the scope ofthis invention.

From a practical standpoint, there is no limit on the molecular weightof the backbone polymer. However, the molecular weight usually rangesfrom about 1×10⁴ to about 1×10⁷. A polymer having a narrow molecularweight distribution is desirable. Conventionally utilized reactions,such as esterification, addition, amidation, etc. can be employed tointroduce the 1,2-ethylene dicarboxylic moiety into the backbonepolymer.

When an amino group is employed as the functional group, a polymerhaving a secondary amino group is contacted with an acid halidecontaining the moiety of the formula (I) at room temperature for severalhours in the presence of a solvent, or with an acid anhydride containingthe moiety the formula (I) at room temperature to about 100° C. forseveral hours in the presence of a solvent. Examples of solvents whichcan be used include ethers (e.g., dioxane), ketones (methyl ethylketone), aromatic solvents (e.g., toluene), etc.

When a hydroxyl group is employed as the functional group, a polymerhaving a hydroxyl group is contacted with an acid halide or an acidanhydride containing the moiety of the formula (I) under the sameconditions as those in the case of the introduction of the amino groupas described above.

When an epoxy group is employed as the functional group, a polymercontaining an epoxy group is contacted with a carboxylic acid directlyor in solution in the presence or absence of a solvent and, if desired,using a catalyst, at a temperature of about 60° to about 120° C.

When an ethylenically unsaturated bond is introduced onto the polymerbackbone, a polymer having an ethylenically unsaturated bond (such as astyrene-butadiene copolymer) is contacted with a carboxylic acidcontaining the moiety of the formula (I) at room temperature to about100° C. for several hours in the presence of a Lewis acid, and a solventsuch as dichloroethane, benzene, toluene, etc.

Examples of the synthesis of the resist material of this invention aregiven below. Unless otherwise indicated, all parts, percents, ratios andthe like are by weight.

SYNTHESIS EXAMPLE 1

10 g. of a styrene-butadiene rubber (SBR); (styrene:butadiene=40:40(molar ratio); molecular weight: 2×10⁻⁶) synthesized by solutionpolymerization was dissolved in 250 ml. of 1,2-dichloroethane in areaction vessel. To the solution of SBR, 2.4 g. of monoallyl maleate and0.4 ml. of borontrifluoride etherate were added. The mixture was stirredfor 5 hours under a dry nitrogen atmosphere. The reaction mixture waspoured into 1 l. of ethanol, and 10.5 g. of the monoallyl maleatederivative of SBR which showed a characteristic I.R. absorption band at1720 cm⁻¹ attributable to a carbonyl group precipitated.

SYNTHESIS EXAMPLE 2

10 g. of the same SBR as described in Synthesis Example 1 above wasdissolved in 250 ml. of 1,2 dichloroethane. To the SBR solution, 2.0 g.of monoethyl fumarate and 0.4 ml. of borontrifluoride etherate wereadded thereto. After the mixture had been stirred for 5 hours at 83° C.under a nitrogen atmosphere, the solution was added to 1 l. of methanolto obtain 10.0 g. of the monoethyl fumarate derivative of SBR which waspale yellow and showed an I.R. absorption of 1720 cm⁻¹ attributable to acarbonyl group.

SYNTHESIS EXAMPLE 3

Distilled methyl methacrylate and 2-hydroxyethyl methacrylate (molarratio=85:15) were added to a glass ampule. Then, 1 wt.% ofazobisisobutyronitrile based on the weight of these monomers was addedwith toluene in an equal weight to the monomers. After the ampule hadbeen evacuated and purged with nitrogen, the ampule was sealed. Theampule was set in an incubator for 24 hours at 60° C. The reactionmixture was dissolved in acetone and precipitated in methanol. Themolecular weight of the polymer obtained was 5×10⁴. 1.0 g. of thispolymer, and 2.9 g. of triethylamine were added to 30 ml. ofN-methylpyrrolidone. To the above mixture, 4.72 g. of monoethyl fumaroylchloride was added dropwise at 5° C. The mixture was then stirred atroom temperature for 12 hours and stirring was continued at 60° C. for 5hours. The reaction mixture was filtered and poured into water to obtain13.0 g. of a brown colored monoethyl furmarate ester of the polymer. Thecrude product was purified by repeating the purification viareprecipitation with N-methylpyrrolidone and methanol as a nonsolvent.

SYNTHESIS EXAMPLE 4

2.0 g. of poly-(1,4-butadiene) comprising 3.3% by weight of thecis-isomer and 53% by weight of the trans-isomer and containing a vinylcontent of 14% by weight and with a molecular weight of 1.6×10⁵, and 4.5g. of monoethyl maleate were dissolved in 50 ml. of 1,2-dichloroethane.To the above solution, 0.9 g. of t-butylhypochlorite was added and thereaction mixture was kept at room temperature for a week. The reactionmixture was added to methanol to obtain 5.2 g. of yellow coloredmonoethyl fumarate derivative of polybutadiene. The polymer showed anI.R. absorption at 1720 cm⁻¹ characteristic of the carbonyl group.

The polymer of this invention having thereon the 1,2-ethylenedicarboxylic moiety can be employed as a resist material formicro-fabrication in the form of a solution in a volatile solvent withan appropriate boiling point.

The volatile solvent dissolves the polymer, and the polymer solution iscoated on the surface of a substrate to form a thin film of the resistmaterial after evaporation of the solvent. For this purpose, the solventshould be capable of being evaporated with heating at a temperatureunder 120° C. to form a thin film and must not be corrosive to thesubstrate on which it is coated. Suitable examples of solvents for theresist material include haloalkanes such as 1,1,1-trichloroethane,dichloroethanes, carbon tetrachloride, etc., esters such as ethylacetate, isoamyl acetate, methyl Cellosolve acetate, alcohols such asethyl alcohol, butyl Cellosolve, etc., ethers such as as dioxane,tetrahydrofuran, etc., and aprotic solvents such asN,N-dimethylformamide, N-methylpyrrolidone, etc. A mixture of two ormore solvents can also be used, if desired.

The examples of suitable volatile solvents given above are for thepurpose of illustration and are not to be considered as limited thepresent invention. The type of polymer used and the solvent employed areselected based on the end-use application.

Also, the concentration of the polymer in the solution and type ofpolymer used will vary depending on the end-use application but normallya suitable polymer concentration is about 5 to 30% by weight.

Next, the application of the resist material of this invention to theultra fine process is described below.

A polymer solution is rotatingly coated on a substrate such as a siliconwater, and if necessary, depending on type of solvent used, the coatedsubstrate is heated to a temperature not more than 200° C. whereby thesolvent is removed.

Using this coating method, a thin film layer of the resist material withthe thickness of less than 30,000 A can easily be formed.

The resist material of this invention is usually used in the form ofthin film layer with a thickness of about 2,000 A to about 30,000 A on asubstrate. The thickness of the resist layer is predetermined tooptimize the fabrication desired. Where the thickness of the thin filmis less than about 2,000 A there is the possibility of undesirablepinhole formation.

With this method, a thin film of the polymer, with a thickness of lessthan about 30,000 A, is formed. If this thin film polymer is irradiatedwith e.g., electron beams, the irradiated portion become insoluble in asolvent and a negative type resist image can be obtained throughdevelopment with an appropriate solvent.

The dose of electromagnetic radiation is generally determined so thatabout 50 to about 100% of the initial thickness of the resist layerafter development remain as the result of crosslinking. Where thepattern made of the resist after development is to be used as the resistfor etching processes, the resist material can be postbaked at about 20°C. to about 250° C. to strengthen the resist.

The resist material of this invention can be employed in producingLSI's, VLSI's (very large scale integrated circuits), surface acousticwave elements, bubble memories and other microelectronic devices.Furthermore, the resist material can be used for mother mask making forphotolithography, X-ray lithography, deep ultra violet lithography, etc.The material also is feasible for producing microfilm, videotape-recording, etc. images.

Some examples of the use of the resist material are described in detailbelow.

EXAMPLE 1

A 15% by weight solution of a polymer produced as described in SynthesisExample 1 in cyclohexanone was filtered through a 0.2 micron membranefilter, and spinner coated (5,000 rpm) onto the surface of a glass platehaving a vacuum deposited layer thereon (dry thickness 2,000 A) anddried for about 15 minutes at 80° C. to form an element having a thinpolymer film of a thickness of 3,000 A thereon. Using a scanningelectron microscope, the element was irradiated with electron beams (27KeV acceleration voltage) in amounts which varied stepwise. Afterirradiation with the electron beams, the element was developed withisoamyl acetate and the thickness of the film remaining was measuredwith an interference microscope. The following results were obtainedD^(i) =3.0×10⁻⁷ coulomb/cm² ; D^(o) =1.6×10⁻⁶ coulomb/cm² ; γ=1.38.

A thin polymer film of a thickness of 3,000 A was formed on theevaporated aluminum surface of another element produced as describedabove, and electron beams (acceleration voltage 10 KeV) were narrowlyfocussed and to 1.5×10⁻¹⁰ coulomb at about 1 cm. After irradiation withthe electron beams, the element was developed with isoamyl acetate toobtain a resist line of a width of 0.7 micron.

After post baking the element for 10 minutes at 120° C., the element wasimmersed for 3 minutes in an etchant solution at 25° C. (phosphoricacid:nitric acid:acetic acid:water:16:1:2:1 by weight).

After washing with pure water, the element was immersed in acommercially available resist removal solution (Tokyo Ohka OMR 501) at100° C. for about 5 minutes and the resist was peeled off. An aluminumline of a width of 0.7 micron was obtained. No side etching was observedas a result of the etching.

EXAMPLE 2

A 15% xylene solution of a polymer produced as described in SynthesisExample 2 was filtered through a 0.2 micron membrane filter, and spinnercoated (4,000 rpm) onto the surface of a silicon wafer having thereon anoxide layer of a thickness of 3,000 A produced by heat oxidation anddried for 10 minutes at 80° C. to obtain an element with a thin polymerfilm of a thickness of 3,500 A thereon.

Using the irradiation procedures as described in Example 1 the elementwas irradiated with electron beams (acceleration voltage of 27 KeV) andthen developed in isoamyl acetate. The results obtained were as follows:D^(i) =7.5×10⁻⁸ coulomb/cm² ; D^(o) =7.2×10⁻⁷ coulomb/cm² ; γ=1.02.

Likewise, a thin polymer film of a thickness of 3,500 A was produced ona silicon wafer as described above, and, in a similar manner asdescribed in Example 1, the electron beam (acceleration voltage: 10 KeV1.0×10⁻⁶ coulomb/cm²) was narrowly focussed on the film surface applyingthe beam in a line, developed in isoamyl acetate and a resist line of awidth of 0.6 micron was obtained.

After post baking for 10 minutes at 120° C., the element was immersedfor 5 minutes into an etching solution (aq. 40% NH₄ F: aq. 56% HF=6:1)at 25° C.

After washing with pure water, the resist was removed using proceduressimilar to those described in Example 1 and a silicon line of a width of0.6 micron was obtained. The adhesion on the resist was excellent andanti-corrosion was good. Side etching and peeling during etching werenot observed.

EXAMPLE 3

A 5% solution in N-methylpyrrolidone of a polymer produced as describedin Synthesis Example 3 was filtered through a 0.2 micron membrane filterand in a similar manner to Example 1 was spinner coated on a vacuumdeposited aluminum surface on a glass plate, dried for 10 minutes at 80°C. and a thin film of the polymer of a thickness of 2,500 A wasobtained. Using the irradiation procedures described in Example 1, theelement was irradiated with electron beams (acceleration voltage of 27KeV) in a stepwise manner. The element was developed in methyl ethylketone and the following results were obtained: D^(i) =1.7×10⁻⁷coulomb/cm² ; D^(o) =1.8×10⁻⁶ coulomb/cm² ; γ=1.03.

EXAMPLE 4

A 10% cyclohexanone solution of a polymer produced as described inSynthesis Example 4 was filtered through 0.2 micron membrane filter, andspinner coated on the vacuum deposited aluminum surface of a glass plateto form a thin film of the polymer of a dry thickness of 3,200 A. Onrepeating the electron beam irradiation procedures as described inExample 1, the following results were obtained: D^(i) =1.8×10⁻⁷coulomb/cm² ; D^(o) =1.2×10⁻⁶ coulomb/cm² ; γ=1.2.

EXAMPLE 5

15 g of a polymer produced as described in Synthesis Example 1 wasdissolved in 100 cc of cyclohexanone, and the resulting solution wascoated on a silicon substrate in the same manner as described in Example1 and prebaked at 80° C. for 30 minutes to form a polymer thin filmhaving a thickness of 5000 A. This thin film was irradiated with softX-rays (Mo L 5.4 A) under a nitrogen atmosphere. The irradiation energywas measured using a gas flow-type proportional counter. The resolutionproperty was measured using a silicon membrane mask having a goldpattern thereon. The polymer thin film was spaced 15000 A from the mask.

After the irradiation with X-rays, the substrate was immersed in methylethyl ketone at 25° C. and then rinsed with dichloroethane, followed bydeveloping the resist. The dose of X-rays (D⁰.5) necessary for 50% ofthe coated film to be insolubilize and to remain after the developmentwas 30 mJ/cm², and the resulting material faithfully reproduced the goldpattern of a pitch of 8000 A. Further, using this thin film as a resist,the silicon substrate was subjected to ion etching with argon ions to adepth of 1000 A, and the resist was removed by ashing in an oxygenplasma, whereby a precise silicon pattern was obtained.

EXAMPLE 6

15 g of a polymer produced as described in Synthesis Example 1 wasdissolved in 100 cc of cyclohexanone. The resulting solution was coatedon a glass substrate having a vacuum deposited layer of chromium thereon(chromium: 1000 A) in the same manner as described in Example 5 anddried to form a thin film having a thickness of 6000 A. The thin filmwas irradiated with a 10 W low pressure mercury lamp at a distance of 10cm through a pattern mask of artificial quartz glass in an nitrogenatmosphere, and then developed in the same manner as in Example 5 toobtain a negative-type image within an irradation period of time of 5minutes. The element was then postbaked at 150° C. for 30 minutes,immersed in a perchloric acid solution of ammonium cerium (IV) nitrate,and etched. The resulting material was washed with water and dried, andthe polymer thin film was swollen with acetone, and then rubbed toobtain a precise chromium pattern.

While the invention has been described in detail and with reference tospecific embodiments thereof, it will be apparent to one skilled in theart that various changes and modifications can be made therein withoutdeparting from the spirit and scope thereof.

What is claimed is:
 1. A resist material for microfabrication consistingessentially of a polymer having a backbone formed of monomer unitsselected from the group consisting of vinyl acetate, vinyl propionate,vinyl chloride, methyl vinyl ketone, vinyl isocyanate, vinyl pyrrolidoneacrylonitrile, ethyleneoxide, alkyl acrylate, alkyl methacrylate,butadiene, isoprene, acrylamide, styrene, itaconic acid and maleicanhydride wherein about 1 to 100 percent of said monomer unit containfunctional group(s) selected from the group consisting of amino groups,hydroxyl groups, epoxy groups and ethylenically unsaturated bonds,bonded to an α,β-unsaturated dicarboxylic moiety of the formula (I)##STR3## wherein R represents a hydrogen atom an alkyl group, an alkenylgroup, an aryl group or an arakyl group, the resist material beingsubstantially non-light sensitive, but curable electromagnetic radiationwith the wave length of less than about 3,000 A.
 2. The resist materialof claim 1, wherein R represents a hydrogen atom.
 3. The resist materialof claim 1, wherein R represents an ethyl group.
 4. The resist materialof claim 1, which is curable by electron beams having an accelerationvoltage of about 10 KeV to 27 KeV.
 5. The resist material of claim 1,which is curable by soft X-rays.
 6. The resist material of claim 1,which is curable by deep ultraviolet radiation having a wavelength ofapproximately 2700 A to 1500 A.
 7. The resist material of claim 1, whichis substantially insensitive to visible light.
 8. The resist material ofclaim 1, wherein said polymer consists essentially of said backbone andsaid α,β-unsaturated carboxylic acid units.
 9. The resist material ofclaim 1, wherein said polymer consists of said backbone and saidα,β-unsaturated carboxylic acid units.